Devices are known for diffusing volatile substances consisting of a container obtained by thermoforming. This type of containers is obtained by joining, on one hand, a thermoformed film, and on the other hand, a preferably flat “diffusion” film through which the substance is released. The space between both films forms the actual container of the substance to be evaporated in liquid, solid (for example gel) form, or impregnated in a porous solid in the container.
The diffusion film has the function of retaining the volatile substance in its liquid or solid form but letting it pass through in its vapor form. It can consist of a semi-permeable membrane, which lets the vapor pass through but is impermeable to liquid, or it can consist of a perforated film which retains a solid but lets its vapor pass through.
The diffusion film is usually covered by an impermeable film which is removed before the first use of the product, and the purpose of which is to prevent the release of the substance during the storage period.
An example of the construction of this type of devices can be found in patents WO9823304A1 or U.S. Pat. No. 6,902,817.
In its simplest embodiment, this type of product has a hook for hanging it in a certain space (a closet rod, the rearview mirror of a car, a metallic structure of a dishwasher, etc).
The possibility of carrying out this type of hook directly in the thermoformed film, which has the advantages of a simpler manufacturing process and a lower cost, is also known.
A known improvement for this type of hook in the thermoformed material is to carry out a mechanical reinforcing embossment during the thermoforming process, whereby obtaining the cavity to be able to offset the low rigidity of the film. An example of this technique is described in U.S. Pat. No. 3,613,994.
This type of devices, however, has the drawback arising when subjected to high temperatures, for example exceeding 70°, which can occur, for example, inside an automobile under the direct impingement of the sun or in the dishwasher. This is primarily because the materials normally used for the thermoforming process are low-temperature processing plastic materials, which are formed at temperatures under 120° C., and even under 100° C. The films to be thermoformed are generally multilayer structures essentially having a core material providing the appearance and the mechanical rigidity to the film, a sealing material for being able to be joined by welding to the diffusion film and other adhesive or barrier layers.
The thermoforming process furthermore usually causes significant residual stress in the same thermoformed film which, when subjected to a certain temperature, causes the film to tend to recover its initial flat shape, causing the distortion of the thermoformed shape. This deformation can be observed at the level of the thermoformed cavity, but it is more critical at the level of the mechanical reinforcing rib of the hook, which can cause the product to fall.
An obvious solution to this problem can be the use of materials having a higher thermal resistance, but this would involve an increase in the process temperatures (which could further be incompatible with the presence of volatile materials in the container) and an increase in the raw material cost.
Another solution could be to use a more robust hook obtained for example by molding, by injection or by manufacturing it out of cardboard. However, this involves an additional process of manufacturing the hook and an assembly process, which involves a significant additional cost. Spanish patent ES-2163668 is an example of this solution.
Hook or hanging systems produced with cardboard, injected plastic or casings injected in plastic with a built-in hook and containing therein the product applied with a membrane are known, and these would also withstand 70° C. but are not carried out with a single process and with only two components.
Therefore, there is a need in the state of the art for a thermoformed container resistant to high temperatures and which can be obtained at a very low manufacturing cost.